The present invention relates to a method for changing the natural frequency of a nip roll construction in a paper board machine and a loading cylinder for nip roll construction in a paper or board machine. The present invention also relates to a loading beam in a nip roll construction in a paper or board machine and a nip roll construction in a paper or board machine, the nip roll construction including two rolls which form the nip.
In paper machines and in paper finishing devices, vibrations constitute quite a considerable problem, and in the present-day systems, in an attempt to achieve ever higher speeds, the problems of vibration have been manifested ever more clearly. There are several possible sources of vibration in paper machines, and some of the most important ones of these sources are rolls and cylinders, which comprise a large mass that revolves at a considerably high speed. Of course, in connection with manufacture, attempts are made to make the dimensional precision of rolls as good as possible and, moreover, the rolls are balanced in order to eliminate vibrations.
In present-day paper machines and paper finishing devices, however, ever increasing use is made of rolls provided with soft coatings, which rolls may constitute a considerable source of vibration during operation. Such rolls are used, for example, in on-line and off-line calenders, coating machines, size presses, supercalenders, and equivalent, in which said roll provided with a soft coating forms a nip with another roll. Through the nip, a paper web and possibly a felt, wire or equivalent are passed. In such a nip roll solution, when a joint or splice of a wire, felt or web, considerable impurities, or something else that causes a considerable change in the thickness of the web passing through the nip runs through the nip during operation, the coating has to yield, in which case the coating acts as a spring that activates vibration. For example, in a size press and in a coating device of the type of a size press, the nip is formed by means of two rolls so that one nip roll has been mounted by means of bearing housings directly on the frame construction of said device, whereas the opposite roll has been mounted by means of its bearing housings on loading arms, which have been linked on the frame construction of the machine. In such a case, in particular, the roll mounted on loading arms starts vibrating, in which connection the coating on the soft-faced roll is deformed, as a result of which the vibration is intensified and the roll starts resonating.
One common prior-art mode of eliminating such vibrations is a change in the running speed of the machine so that, at the running speed concerned, the vibration is no longer intensified but starts being attenuated. Thus, problems of vibration have restricted the running speed of the machine.
In the applicant""s FI Patent 82,127, a method and a device are described for attenuation of vibration of rolls, in particular in a pair of rolls that form a nip in a press section. In the method, the nip is run into the closed position and to the desired linear load of running so that the load produced by the loading devices of the pair of rolls is stabilized to a regulated level. After the desired linear load of running has been reached and after the load produced by said loading means has been stabilized, the bearing housings of the rolls constituting the pair of rolls are coupled with each other rigidly so that their relative movement is prevented.
In the applicant""s FI Patent 85,166, a method and a device developed further from the above FI Patent 82,127 are described for attenuation of vibration of rolls that form a nip, in particular in a pair of rolls that form a nip in a press section. In the method, the bearing housings of the rolls in the pair of rolls are supported against each other by means of an attenuator device, by whose means relative movements of the rolls that form the pair of rolls, which movements arise from vibration, are attenuated and absorbed. The attenuation is carried out by means of a hydraulically operating attenuator device which comprises a piston-cylinder construction and into which a hydraulic pressure medium is passed and out of which said pressure medium is removed. By means of the attenuator device, the flow of hydraulic pressure medium caused by the relative movement in the piston-cylinder construction arising from the rolls in the pair of rolls is intensified, and said intensified flow is throttled in order to produce attenuation of vibration.
In the applicant""s FI Patent 94,458, a method and an equipment are described, by whose means the critical speed of a roll can be changed in view of prevention of vibration. The critical speed of a roll can be changed by changing the mass of the roll and/or by changing the rigidity of the suspension of the roll and/or by changing the location of the roll suspension point in the axial direction of the roll and/or by changing the elastic constant of the journalling of the roll and/or by supporting the roll from the roll face by means of a displaceable support roller.
In the applicant""s FI Patent Application 971864, a method is described for attenuation of oscillation in a paper machine or in a paper finishing device by means of a dynamic attenuator, which comprises an additional weight suspended on the oscillating object by means of a spring. In the method, the oscillation frequencies of the oscillating object are measured constantly by means of one or several oscillation detectors. The measurement signals given by the oscillation detector are amplified by means of an amplifier and fed into an oscillation analyzer, which identifies the problematic excitation frequency and converts said problematic excitation frequency into a control signal. The control signal is fed into a control device, by whose means the elastic constant of the spring of the dynamic attenuator and/or the mass of the dynamic attenuator is/are changed in order to make the specific frequency of the dynamic attenuator substantially equal to the problematic excitation frequency. The dynamic attenuator may consist, for example, of a substantially horizontal bar attached to a bearing housing of the roll, an additional weight being suspended on said bar and the location of the weight on the bar being displaceable.
The method and the device in accordance with the invention are particularly well suited for use in a nip roll construction in which a roll nip is formed between a soft-faced roll and a hard-faced roll. In such constructions, rapidly increasing vibrations may arise through deformation of the coating on the soft-faced roll, and it must be possible to bring such vibrations under control.
This is accomplished in the present invention so that the rigidity, and so also the natural frequency, of the nip roll construction is varied before the deformation has had time to develop too far, in which case it is possible to keep the level of vibration under control. The rigidity of the nip roll construction can be varied so that the rigidity of one or several loading elements that form the nip roll construction is varied. In the invention, the rigidity of the nip roll construction is influenced by means of a loading cylinder whose rigidity can be regulated and/or by means of a loading beam whose rigidity can be regulated, each of which (cylinder and/or beam) forms one loading element in the nip roll construction. In the invention, the rigidity of a loading element or elements is varied while the linear load in the nip or the rest of the operation of the nip is not influenced. Thus, regulation of the rigidity of a loading element has no effect on the properties of the paper running through the nip.
The regulation of the rigidity of the loading element can be carried out in an oscillating way, in which case the regulation does not have to be connected with measurement of the vibrations of the nip roll construction. The regulation of the rigidity of the loading element can also be made automatic. In such a case, the vibration frequencies of the vibrating object are measured continuously by means of one or several oscillation detectors. The measurement signals given by an oscillation detector are amplified by means of an amplifier and fed to an oscillation analyzer, which identifies a problematic excitation frequency and converts said problematic excitation frequency into a regulation signal. The regulation signal is fed into a regulation device, by whose means the rigidity of one or several loading elements of the nip roll construction is varied.